Rotary offset printing apparatus comprise an inking device designed to distribute as regularly as possible a film of ink on the peripheral surface of an inking roller itself in contact with a plate cylinder. Heretofore known inking devices generally comprise an ink fountain containing a mass of ink and a succession of horizontal rollers of parallel axes, to transfer the film of ink from the ink fountain to the surface of the inking roller. These rollers comprise, from upstream to downstream, firstly an ink fountain roller driven in rotation at low peripheral speed, this ink fountain roller defining, on one side, the ink fountain. The ink fountain roller takes along on its periphery a film of ink whose thickness is adjustable and is fixed by the position of a doctor blade disposed in the immediate vicinity of the peripheral surface of the ink fountain roller, in the lower part of the descending movement of the generatrices of this roller. The film of ink taken from the surface of the ink fountain roller is then transferred to the surface of a first roller, also called inking table, of a succession of rollers tangential to one another and finally depositing the film of ink on the periphery of the inking roller. The film of ink is generally transferred between the ink fountain roller and the inking table by means of a taker roller which is disposed between the ink fountain roller and the inking table. This taker roller is mounted idly on an oscillating support so as to be sometimes tangential to the ink fountain roller, in order to take the film of ink from the surface thereof, sometimes tangential to the inking table in order to deposit the film of ink thus taken on the surface of this inking table.
Among the inking devices of the type set forth hereinabove, certain are known which comprise an additional transfer roller making the inking device operate, as desired, either in accordance with a conventional inking mode with taker roller animated by an alternative movement, or a pellicular inking mode in which the taker roller is permanently maintained in contact with the inking table and the additional transfer roller, which is normally spaced apart during the inking mode employing alternative taker roller and which is brought to a very short distance from the ink fountain roller and in contact with the taker roller. Consequently, the film of ink taken by the ink fountain roller is transferred, by the additional transfer roller, to the taker roller and from the latter, to the inking table. Such inking devices with two operational modes are described for example in Patents DE-B-2 703 424 and EP-A-0 035 735.
In the inking device according to Patent EP-A-035 735, due to the fact that, in the pellicular inking mode, the ink fountain-, additional transfer-, taker rollers and the inking table are tangential to one another in that succession, the inking table necessarily rotates in direction opposite the ink fountain roller since there exist therebetween two intermediate rollers, namely the additional transfer roller and the taker roller. Consequently, when the inking device operates in alternative ink taking mode, i.e. when the additional transfer roller is in inactive position, spaced apart from the ink fountain roller and the taker roller, and this taker roller oscillates so as to be alternately in contact with the ink fountain roller and the inking table, the taker roller alternately encounters, during each oscillation, the surface of the ink fountain roller, rotating in one direction at very low speed, and that of the inking table, rotating in opposite direction at greater speed. Consequently, during each oscillation, the idle taker roller which was driven in rotation at high speed upon its previous contact with the inking table, then comes into contact with the ink fountain roller which, since it rotates in the same direction as it, moves in opposite direction in the zone of contact at much lower peripheral speed. The taker roller is then braked until it stops, and is then returned in direction opposite the preceding one, before coming into contact again with the inking table where, there again, it undergoes a braking then a reversal of its direction of rotation. Such an operation obviously presents the drawback of being detrimental to the mechanical elements of the inking device and it is an essential object of the present invention to overcome this drawback.